A class of adhesive compositions known as thermoplastic or hot melt adhesives have enjoyed continually increasing usage in many industrial applications. These adhesives, or hot melts, as they are commonly designated are solid or semi-solid combinations of film-forming resins, tackifying resins, rubbery polymers, plasticizers, waxes and similar materials which are added to the adhesive composition in order to impart various properties thereto.
Adhesive bonds derived from hot melts are particularly useful because (1) they are 100% solid materials and (2) they quickly form tough adhesive bonds simply by cooling--i.e., no need for solvent evaporation, etc.
Adhesives derived from hot melts are particularly useful because of their very rapid set up or bonding time. Strong bonds are formed when the adhesive cools below its melt point and solidifies. There is no long wait for solvent or water evaporation or for chemical curing as with other types of adhesives. Depending on composition, hot melt adhesives can be prepared having good tack, high bond strength, good flexibility, low temperature properties, good environmental resistance, etc.
A typical class of hot melt adhesive composition utilizes polyolefin polymers as the base or carrier material. The polyolefin base is usually blended with other polymers and copolymers, resin tackifiers, and with modifiers and additives. Generally, the polyolefin carrier is polypropylene (either atatic or crystalline or both), EVA, or polyethylene, but that is usually done for cost considerations and it is not necessary that it be so restricted from a technical standpoint.
In spite of the tremendous growth in the use of hot melts and the advantages to be realized by their use, the extent of their utilization has nevertheless been limited by several serious shortcomings. Polyolefin based hot melts, for example, usually suffer from the lack of strong adhesion to nonprous metallic surfces, such as aluminum, steel and the like. They also tend to be quite brittle unless suitably modified. All hot melts generally suffer from low bond strengths at elevated temperatures. When heated, even well below their melt points, they tend to soften and lose strength.
The low temperature brittleness and high temperature softening difficulties of previous hot melts have been approached by changing to epoxy adhesives to which are added elastomers to make rubber toughened epoxies. However, it would be better to achieve similar properties by the addition to cross-linking agents to the relatively less expensive olefin polymers.
Another type of useful adhesive system is the solution based adhesive including pressure sensitive adhesive formulations having multiple uses.